1. Field of the Invention
The present invention generally relates to an anti-skid control apparatus for a motor vehicle which includes an acceleration sensor for detecting acceleration of the motor vehicle for utilization of the acceleration information in the anti-skid control. More particularly, the invention is concerned with an improved anti-skid control apparatus which is essentially insusceptible to the influence of an offset quantity usually applied to the output of the acceleration sensor in order to compensate for variances in the characteristic thereof. Furthermore, the present invention is concerned with an apparatus for detecting acceleration of a motor vehicle by using a conventional acceleration sensor, which apparatus can be used not only for the anti-skid control but also for other purposes.
2. Description of the Related Art
For a better understanding of the invention, description will first be directed to the related or background techniques.
FIG. 10 is a schematic block diagram for illustrating a structure of a typical one of the anti-skid control apparatuses known heretofore.
Referring to the figure, the anti-skid control apparatus includes as a major part a controller 1 comprised of a microcomputer 5 which incorporates therein a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM) and others. As the peripheral devices of the controller 1, there are provided an acceleration sensor 2 for detecting acceleration of a motor vehicle (not shown) which is equipped with the anti-skid control apparatus under consideration, and wheel speed sensors 3, 6, 8 and 10 for detecting velocities or speeds of the individual wheels (not shown) of the motor vehicle for generating wheel speed signals each of a sinusoidal waveform and having a frequency varying in proportion to the wheel speed as detected. The detection signals outputted from these wheel speed sensors 3, 6, 8 and 10 are supplied to associated waveform shaping circuits 4, 7, 9 and 11, respectively, each of which serves to convert the detection signal supplied from the associated wheel speed sensor into a pulse signal, which is then supplied to the microcomputer 5. The waveform shaping circuits 4, 9, 7 and 11 are incorporated in the controller 1. Further, the controller 1 includes current control circuits 12, 15, 17 and 19 for controlling currents supplied to solenoid valves 13, 16, 18 and 20, respectively, of a hydraulic brake system in accordance with current control commands issued by the microcomputer 5. All the solenoid valves 13, 16, 18 and 20 are electrically connected to a common power supply source 14.
For clarifying the problem which the invention is to solve, description will be made of the characteristic of the acceleration sensor 2 by reference to FIG. 11 which shows in a characteristic diagram a typical characteristic of the acceleration sensor 2. Referring to FIG. 11, the detection voltage VG outputted from the acceleration sensor 2 and indicating the acceleration of the motor vehicle body is taken along the ordinate with the actual vehicle body acceleration being taken along the abscissa. In this figure, a solid line curve represents an ideal characteristic while a broken line curve represents a characteristic which is offset from the ideal characteristic by an offset quantity or voltage v. In the case of the acceleration sensor 2 having the ideal characteristic, the detection voltage VG changes linearly in proportion to the actual vehicle acceleration and exhibits a predetermined voltage value VGS when the vehicle acceleration is zero. Further, in the case of the illustrated example, the detection voltage VG is offset by the quantity v of minus sign. Parenthetically, magnitude of the offset voltage v is determined in dependence on the characteristic of the acceleration sensor actually installed as well as time-dependent tendency of change because these factors will differ from one sensor to another.
Next, description will turn to control operations performed by the microcomputer 5 of the known anti-skid control apparatus by reference to a flow chart of FIG. 12.
The acceleration sensor 2 is adapted to detect acceleration of the motor vehicle in the state where the vehicle is being driven. The detected vehicle body acceleration signal is supplied to the microcomputer 5 as the input data. At the same time, the wheel speed sensors 3, 6, 8 and 10 generate the sinusoidal waveform signals having frequencies changing in dependence on the velocity or speeds of the associated wheels, respectively, which signals are converted to corresponding pulse signals through the waveform shaping circuits 4, 7, 9 and 11, respectively, and then supplied to the microcomputer 5.
In a step S1, the microcomputer 5 calculates the wheel speeds on the basis of the wheel speed signals as inputted, whereupon the processing proceeds to a step S2. More specifically, in the step S1, the microcomputer 5 arithmetically determines the periods of the wheel speed pulse signals supplied from the waveform shaping circuits 4, 7, 9 and 11, respectively, by activating a corresponding interrupt processing (not shown), to thereby calculate the wheel speeds VW in terms of the reciprocals of the periods as determined.
In the step S2, differences between the wheel speeds VW determined currently in the step S1 and the wheel speeds determined in the preceding cycle are calculated to thereby obtain the wheel accelerations GW. Then, the processing proceeds to a step S3 in which the detection voltage VG generated by the acceleration sensor 2 is converted into digital data through an analogue-to-digital (A/D) converter (not shown), the digital data being fetched by the microcomputer 5. Then, the processing proceeds to a step S4.
In the step S4, the vehicle acceleration GB is determined on the basis of the digital data mentioned above, whereupon decision is made whether a brake oil pressure P of a hydraulic brake system (not shown) is to be increased, decreased or alternatively to be held at a current value on the basis of the vehicle acceleration GB, the wheel speeds VW and the wheel accelerations GW in accordance with a predetermined algorithm (not shown).
In the step S5, the microcomputer 5 outputs current command values to the current control circuits 12, 15, 17 and 19, respectively, in accordance with the results of the decision step S4, whereby the currents of the corresponding values are supplied to the solenoid valves 13, 16, 18 and 20, respectively, from the power supply source 14, as a result of which the brake oil pressures P for the hydraulic brakes (provided in association with the wheels, respectively) are increased, decreased or alternatively held at the respective current levels. In this manner, the anti-skid control is effectuated. The routine including the processing steps S1 to S5 mentioned above are executed cyclically or periodically at a predetermined time interval TL.
Next, referring to a waveform diagram shown in FIG. 13, operation of the anti-skid control apparatus shown in FIG. 10 will be elucidated in detail. In this figure, a solid line curve A represents the detection voltage signal VG generated by the acceleration sensor 2, a solid line curve Ba represents an actual vehicle speed, a broken line curve Bb represents the Vehicle speed VB estimated on the basis of the output of the acceleration sensor 2, a solid line curve Bc represents the wheel speeds VW, a solid line curve C represents changes in the brake oil pressure, and a pulse waveform D represents the current command signal issued by the microcomputer 5 in which a pressure reduction-command is indicated by a pulse of relatively long duration, a pressure increase command is represented by a series of short pulses, and a hold command is represented by a base line.
Now, let's assume that the brake pedal is actuated or depressed in the course of driving the motor vehicle. Then, the braking oil pressure P for the wheels increases steeply, as can be seen from the curve C. When the braking force exceeds or overcomes the friction between the ground surface and tires of the wheels, the latter will immediately transit to the locked state. At that time, the wheel acceleration signals GW assumes a large value of minus sign (i.e., deceleration), and magnitude of slip or skid (given in terms of differences between the vehicle speed VB and the wheel speeds VW) increases steeply. The microcomputer 5 detects occurrence of the locked state of the wheels on the basis of the wheel decelerations GW and the slip to issue to the current control circuits 12, 15, 17 and 19 current commands for decreasing the brake oil pressure P. As a consequence, the brake oil pressure P is decreased under the actions of the solenoids 13, 16, 18 and 20, whereby the wheels are restored or released from the locked state. Thus, the wheel acceleration signals GW shifts from the minus region (deceleration) to the plus region (acceleration), as a result of which tendency for occurrence of slippage is mitigated.
When the wheels are released or restored from the locked state, the microcomputer 5 detects this event on the basis of the wheel acceleration signals GW and the change in the slip and issues the current hold command to the current control circuits 12, 15, 17 and 19, as a result of which the brake oil pressure P effective currently is held under the action of the solenoids 13, 16, 18 and 20.
When the wheels are restored substantially completely from the lock tendency with the wheel speeds approaching sufficiently to the vehicle speed VB, the brake oil pressure increases only, progressively, because the microcomputer executes repeatedly the processing for issuing the current command to increase the brake oil pressure only for a short time and then the current level hold command. Consequently, the brake oil pressure P increases progressively, as indicated by stepwise progressive changes in the curve C. Eventually, the friction between the ground surface and the wheels is exceeded by the braking force, where by the wheels are again locked, whereupon the microcomputer 5 issues the current command for decreasing the brake oil pressure to the current control circuits 12, 15, 17 and 19. By repeating the control procedure described above, the brake oil pressure for the wheels is so controlled that the braking force for the wheels lies in the vicinity of a value indicating maximal friction between the road surface and the tires of the wheels.
It should here be mentioned that in the anti-skid control apparatus of the type described above, availability of the vehicle speed VB is indispensable. However, it is very difficult from the technical standpoint to detect straightforwardly the vehicle speed signal. Under the circumstances, the vehicle speed VB is estimated by resorting to various methods.
Again referring to FIG. 13, a method of estimating or calculating the vehicle speed VB will be described. So long as there exists no tendency of the wheels being locked, the wheel speed VW may be considered to coincide at least approximately with the actual vehicle speed. Accordingly, the vehicle speed VB can be estimated directly from the wheel speed VW. On the other hand, when the wheels tend to lock, the vehicle speed VB is decreased in conformance with a gradient corresponding to the vehicle acceleration GB. In other words, the vehicle speed VB can be determined by integrating the vehicle acceleration GB. When the wheels get free of the lock tendency and reaches the speed level corresponding to the vehicle speed VB, the latter can be determined directly from the wheel speeds VW. The vehicle speed is determined by executing repetitively the series of operations described above.
At this juncture, it should be mentioned that a method of determining indirectly the vehicle speed VB by integrating the detection voltage signal VG generated by the acceleration sensor is disclosed in Japanese Unexamined Patent Application Publication No. 77352/1990 (JP-A-H2-77352).
The anti-skid control apparatus known heretofore and implemented in the configuration described above suffers problems mentioned below. First, because the characteristic of the acceleration sensor is not always ideal, the vehicle speed determined by integrating the detection signal outputted from the acceleration sensor will unavoidably contain error more or less, which presents an obstacle to realization of the anti-skid control in a satisfactory manner. Inherently, the anti-skid control is so designed as to be effective on a road surface of small friction. Consequently, when the offset (i.e., the output of the acceleration sensor when the vehicle acceleration is zero) is set at a large value with a view to compensating for the variances mentioned above, the anti-skid control will become effective even in the situation where the anti-skid control is not required. The problems mentioned above can be ascribed to the fact that with the conventional vehicle acceleration sensor, it is difficult or impossible to detect the acceleration of the motor vehicle with sufficiently high accuracy for practical applications such as anti-skid brake control of the motor vehicle.